For this he is highly regarded by Republicans, but at the same time equally reviled by Democrats.

"Obviously, it's a big loss to us," White House deputy press secretary Dana Perino told the Associated Press news agency. "He's a great colleague, a good friend, and a brilliant mind. He will be greatly missed."

"He will continue to be one of the president's greatest friends," she added.

Delayed departure

Mr Rove has been accused of underhand political tactics since his teenage years.

HAVE YOUR SAY

If Rove has been Bush's chief advisor, he has given a lot of very very bad advice.

Colin Powell, secretary of state, resigned after President Bush's first term, Nov 2004

SURVIVORS FROM 2000

Dick Cheney, vice-president

Condoleezza Rice, secretary of state, was national security adviser in first term

Alberto Gonzales, attorney general, was White House counsel

Mr Rove was also investigated in connection with the exposure of CIA agent Valerie Plame, though prosecutors decided he should not face any charges.

Mr Rove told the Wall Street Journal that he had first floated the idea of leaving last year, but had delayed his departure when the Democrats took control of Congress.

He said he took a final decision to leave after White House Chief of Staff Joshua Bolten told aides that if they stayed after the end of August they would be obliged to stay in the administration until Mr Bush's own departure in January 2009.

"There's always something that can keep you here, and as much as I'd like to be here, I've got to do this for the sake of my family," he said.

He said he expected Mr Bush's current poor ratings to improve, and that conditions in Iraq would get better as the military surge continued.

A Republican had a good chance of winning the 2008 presidential election, he said, because Democrats would choose the "fatally flawed" Hillary Clinton as their candidate.